a. The Field of the Invention
This invention relates to the field of network systems. In particular, the invention relates to a systems for reducing electromagnetic interference emissions in network interfaces.
b. Background Information
Ethernet is an industry standard (e.g., IEEE 802.3 specification) method of communicating between various devices and a local area network. For example, a computer includes a network interface card (NIC) that formats Ethernet data for transmission onto a network cable. The network cable carries the Ethernet formatted packets out to the rest of the network. The data signal is generated to comply with a particular specification for that type of Ethernet communications. For example, the Ethernet data signal might be generated to comply with the ANSI/IEEE standard 802.3 Ethernet voltage template. This voltage template applies to five and ten MHz frequency components of Ethernet data communications. Complying with the voltage template ensures that the NIC will not damage other devices connected to the cable, and ensures that other devices will be able to properly receive and decode the signals from the NIC.
One prior art NIC is shown in FIG. 1. This NIC is available from 3COM Corporation, of Santa Clara, Calif. The NIC 100 is for generating the transmit signal 130 which corresponds to an Ethernet transmit signal. The transmit signal 130 is generated to support Manchester encoding of the Ethernet data. The transmit signal 130 is a combination of two other signals, a data signal and a pre-emphasis signal. The pre-emphasis signal adds a slight step to some of the waves in the transmit signal 130.
The transmit signal 130 is created from four output signals from the Ethernet controller 101. The Ethernet controller 101 is responsible for generating the pre-emphasis signal and the data signal. The combinations of these signals is eventually transmitted as the transmit data plus (TDP) 112 and the transmit data minus (TDM) 114. Between the Ethernet controller 101 and the transformer 120 is a filter circuit that filters and combines the four output signals from the Ethernet controller 101 into the transformer 120. (The transformer 120 is for electrical isolation and includes a seven pole filter.) The transformer 120 is available from Valor Corporation.
As part of the FCC's electromagnetic interference regulations, the NIC 100 must not emit an amount of electromagnetic radiation above a preset limit. The FCC and CISPR-B specifications, for example, limit the radiation from the NIC 100. Importantly, any harmonic output above the fundamental frequency must be less than twenty-seven DB below the output at the fundamental frequency. Therefore, it is desirable to be able to reduce the amount of these high frequency components.
One problem with the NIC 100 is that the environment in which the NIC operates varies considerably (e.g., the temperature changes, the load on the wire changes, variations in the manufacturing processes, power supply variations). This variation can result in a change in the transmit signal 130. For example, the temperature of the NIC 100 can significantly change the signal strength of the transmit signal 130. The variation also arises as a result of manufacturing differences between different NICs 100. This variation is undesirable in that it is difficult to meet the electromagnetic interference specification requirements while mataining the desired output levels for the transmit signal 130. Therefore it is desirable to have some form of control over these output values.
Also, it is desirable to reduce the filtering requirements after the output of the Ethernet controller 101. Reducing the filtering requirements can reduce the component count of the NIC 100. This may significantly reduce the manufacturing costs of the NIC 100.